1. Technical Field
The invention relates to induction heating and an improved induction furnace. More particularly, the invention relates to an induction furnace for melting materials not susceptible to inductive heating at lower temperatures but which are susceptible to inductive heating at higher temperatures, especially upon melting. Specifically, the invention relates to an induction furnace capable of continuously or intermittently melting such materials.
2. Background Information
Induction furnaces are well known in the art. However, there are a variety of difficulties related to the inductive heating and melting of materials that are initially non-conductive or which have particle sizes sufficiently small so that they are not susceptible to inductive heating. Many prior art induction furnaces utilize a conductive crucible such that an induction coil couples with the crucible to transfer energy directly to the crucible to heat the crucible whereby heat is then transferred from the crucible to the material to be melted via thermal conduction. In certain cases, the induction frequency and the thickness of the crucible wall may be selected so that a portion of the electromagnetic field from the coil allows coupling with electrically conductive material inside the crucible to inductively heat the material directly. However, the direct inductive heating in such cases is quite limited. Because direct inductive heating of the material to be melted is far more effective than the method described above, a system to effect such direct inductive heating is highly desirable.
In addition, the conductive crucibles of the prior art may react with the material to be melted which causes unwanted impurities in the melt and thus requires the use of a non-reactive liner inside the crucible to prevent formation of such impurities. Typically, however, such liners are electrically non-conductive and thermally insulating. As a result, the transfer of heat from the crucible to the materials to be melted is greatly impeded and thus melting times are substantially increased. To expedite the transfer of heat from the crucible to the material to be melted, the crucible must be heated to undesirably high temperatures which can decrease the life of the crucible and liner.
In addition, there remains a need for an induction furnace capable of producing a continuous melt in an efficient manner, especially for semi-conductor materials. An efficient continuous melt induction furnace is particularly useful related to continuous formation of semi-conductor crystals, which are highly valued in the production of computer chips.
U.S. Pat. No. 6,361,597 to Takase et al. teaches three embodiments of an induction furnace especially intended for melting semi-conductor materials and adapted to supply the molten material to a main crucible for pulling of semi-conductor crystals therefrom. Unlike the prior art discussed above, Takase et al. uses a quartz crucible which is electrically non-conductive along with a susceptor which is in the form of a carbon or graphite cylinder. In each of the three embodiments of Takase et al., the carbon or graphite cylinder susceptor is initially inductively heated by a high frequency coil whereby heat is transferred from the susceptor to raw material inside the crucible in order to begin the melting process. Once the raw material is melted, it is directly inductively heated by the high frequency coil in order to speed up the melting process. While this is a substantial improvement over the previously discussed prior art, the induction furnace of Takase et al. still leaves room for improvement.
The first two embodiments of Takase et al. involve the use of a carbon cylinder susceptor which encircles the quartz crucible and is movable in a vertical direction. This provides a mechanism whereby the susceptor may be inductively heated and then either moved out of the electromagnetic field of the induction coil altogether or moved to a position which is more advantageous for heating selected portions of the material within the crucible. One drawback of this configuration is the need for a mechanism to move the susceptor in a vertical direction. The third embodiment of Takase et al. provides a susceptor having a crucible-like configuration with a cylindrical side wall of the susceptor covering the side wall of the quartz crucible and a bottom of the susceptor covering the bottom wall of the quartz crucible. The susceptor is not vertically moveable in the third embodiment. Instead, the thickness of the susceptor sidewall and the frequency applied by the coil are selected so that the penetration depth of the induction current will extend beyond the susceptor into the quartz crucible so that it can inductively heat material inside.
The third embodiment of Takase et al. primarily suffers from the fact that the cylindrical susceptor remains in place and thus prevents inductive heating from more effectively being focused on the raw material within the crucible. Instead, the coil continues to inductively heat the carbon cylinder so that energy which might be applied to the material is absorbed by the carbon cylinder, which transfers heat to the raw material in the crucible in a far less effective manner.